Various processes and procedures are used to fabricate integrated circuits on semiconductor wafers. Examples of these processes include oxidation of silicon to form a silicon dioxide (SiO.sub.2) film on a silicon substrate and microlithography which generally involves placing a radiation sensitive material (sometimes referred to as a "photoresist" or "resist") on the oxide film. Structural features of the integrated circuit are frequently developed on the wafer by either depositing material, oxidation, or removing the underlying material (etching). Typically, the resist is radiated through a mask placed over the resist by lithography techniques such as photolithography, electron beam lithography or x-ray lithography. The resist reacts chemically in the exposed areas of the mask depending upon the choice of the chemical system.
Other processes commonly associated with semiconductor wafer fabrication include diffusion of controlled impurities or dopants into the films. Ion implantation is also used to introduce controlled impurities into the films. Additional fabrication processes include physical vapor deposition of thin films by sputtering and physical vapor deposition of thin films by evaporation.
The sequence of wafer fabrication processes and procedures are selected depending upon the type of semiconductor wafer and the desired operating characteristics of the integrated circuit which will be fabricated on the semiconductor wafer. Fabrication processes are frequently repeated several times to build up multiple layers associated with modern very large scale integrated circuits which are fabricated on semiconductor wafers.
Etching is a commonly used process in the fabrication of many integrated circuits on semiconductor wafers having a silicon substrate. Etching involves removal of material from the silicon substrate and/or selected thin film layers which were previously deposited on the surface of the silicon substrate. When a mask is used to protect portions of the wafer surface, the goal of etching is to remove material which is not covered by the mask. The primary objective of the etching process is to precisely transfer the pattern established by the protective mask onto the underlying material.
Various techniques such as ellipsometry are used to measure film thickness on semiconductor wafers. Ellipsometry also measures the optical index of refraction. Ellipsometry techniques are frequently used to measure the thickness and index of refraction of various types of dialectic films and also determine the thickness of multiple layer film structures such as polysilicon on silicon dioxide or a photoresist on silicon dioxide. Other techniques for determining film thickness include optical interference, capacitance methods and/or oxide film color charts.